util: Replace non-threadsafe strerror

Some uses of non-threadsafe `strerror` have snuck into the code since
they were removed in #4152. Add a wrapper `SysErrorString` for
thread-safe strerror alternatives and replace all uses of `strerror`
with this.

src/fs.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <fs.h>
+#include <util/syserror.h>
 
 #ifndef WIN32
 #include <cstring>
@@ -44,7 +45,7 @@ fs::path AbsPathJoin(const fs::path& base, const fs::path& path)
 
 static std::string GetErrorReason()
 {
-    return std::strerror(errno);
+    return SysErrorString(errno);
 }
 
 FileLock::FileLock(const fs::path& file)